How Ants Create "Supersoldiers"—And Why More Species Don't

Across a broad group of ants, just eight species were known to naturally create freakishly huge "supersoldiers" to defend the colony. But new research shows that many more ants have the genetic building blocks to create these defenders—and that if times get tough, they might do it.

In a select few species of ants, the familiar caste system that divides members in classes like workers or soldiers has another level: supersoldier. These huge insects fight off invading army ants, often by using their extra-large heads in a Spartan-like defense.

The supersoldier caste appears in just eight species of the hyper-diverse Pheidole genus of ants. But researchers have discovered, in a new study released today in Science, that a group of more than 1000 species of ants have retained hidden ancestral genetic material to build supersoldiers—they're just not using it. The findings suggest that the ability to create these giant ants evolved in a common ancestor of all Pheidole species.

Lead researcher Rajee Rajakumar, a Ph.D. student at Montreal's McGill University in the lab of Ehab Abouheif, got curious about why so few ant species can make supersoldiers a few years ago when he discovered on Long Island a wild P. morrisi colony, a species of the Pheidole genus, with unexpected supersoldier-like features. The discovery suggested Pheidole species share a developmental origin when it comes to supersoldiers, since the eight species with this type of ants are found only in the deserts in southwest America and northern Mexico.

"Whatever selected pressures are unique to Arizona, they're not there in Long Island, yet they still produce these low-frequency anomalies," Rajakumar says. "With that observation, we came to the conclusion that maybe all of these species in this group actually have in their genome this ancestral potential to produce supersoldiers, even though they don't."

When ant larvae develop, which takes several weeks, embryos can become minor workers, which perform the majority of tasks of the nest; soldiers, who defend the nest and process food; and supersoldiers. Environmental factors can change the outcome. So, working with University of Arizona researchers, Rajakumar tried to induce P. morrisi ant larvae to grow into supersoldiers by applying the growth-spurring juvenile hormone to the larvae. It worked, indicating that environmental cues can turn on the genetic mechanism that produces the supersized ant.

"Supersoldiers really seem to be something that has laid dormant from the ancestor of all of these species of Pheidole," he says.

So why haven't all Pheidole species evolved to regularly create supersoldiers? One explanation is the evolution of an alternate defense strategy. For instance, one species in this group of ants, called P. hyatti, responds to army ant raids by evacuating the entire nest and recolonizing elsewhere. Yet in the lab, Rajakumar and his team were able to produce supersoldiers in this species as well, bolstering evidence that this trait exists in the genetic building blocks of all Pheidole species.

Just why supersoldier-like ants cropped up in Long Island remains unclear. But fellow researcher, Ming Huang, wrote in an email that the environment plays a crucial role. A shift in climate or a change in nutrition could result in species' production of supersoldiers, the appearance of which dates back 35 million to 65 million years in the Pheidole genus.

For some species, calling on ancestral tool kits could mean survival. "Maintaining ancestral developmental tools allow the organisms to re-evolve the traits relatively quickly if their living conditions change," says Ming, a Ph.D. candidate at the University of Arizona's entomology department.

Corrie Moreau, an evolutionary biologist at the Field Museum of Natural History in Chicago, says Rajakumar's study uncovers key information about the ant tree of life, which she has reconstructed in her own research. "What's striking is that there's a developmental pathway that's been around for millions of years and hasn't been used all that often," Moreau says.

Rajakumar's study of developmental pathways through hormone treatments is novel in ant research, she says, which doesn't see many evolutionary biology studies. "We have 12,000 species of ants," Moreau says. "They've only looked at one lineage that contains 1000. So the question is, can we use the same method across the ant tree of life?"

Huang argues that the answer is yes: Further research will reveal more ants with the potential to create supersoldiers, and other hidden developmental traits, buried in their DNA.

"Our discovery of a hidden ancestral potential for the supersoldier trait in Pheidole ants means that this phenomenon can be occurring in other ants, other insects and other organisms," he says.